A membrane material is a semi-permeable physical barrier having the property of differentiation between the rates of transport of different components of a fluid mixture. The selective transport of components of the fluid mixture at different rates is generated by the gradients of driving forces, such as pressure, partial pressure and temperature. Selective transport basically results in the dividing of the raw fluid mixture into retentate and permeate portions. The retentate portion is enriched with slowly permeable components of the raw mixture, whereas the permeate portion is enriched with components of the raw mixture which migrate faster.
The membrane separation process is typically carried out in a module fabricated from elements of the membrane material. The module typically has a feed inlet leading to a first flow passage on one side of the membrane material of the elements, a retentate outlet leading from the first flow passage, and a permeate outlet leading from a second flow passage on an opposite side of the membrane material of the elements across which the fluid separation process occurs. The fractions of raw mixture leaving the retentate and permeate outlets of the membrane module are the products of the separation process occurring therein as a result of the supply at a higher pressure through the feed inlet of the module of an input stream of the raw fluid mixture to be separated.
The efficiency of the fluid separation process is determined by the properties of the input stream of raw mixture and of the membrane material and its structure. Generally, the fluid separation process exhibits a complex physiochemical mechanism. The productivity of the membrane module increases as the surface area of the membrane material packed in the membrane module increases. Separation efficiency of the module depends inversely on the thickness of the membrane material.
High packing density of membrane surface area inside of the module is realized most frequently by providing the elements of membrane material as hollow fibers of substantial length arranged parallel to one another. The most logical way to increase the packing density of the membrane surface area in the module is to decrease the diameter of the hollow fibers. However, decreasing the diameter of long fibers increases the problem of backpressure due to the resistance of their internal channels to the inflow of the raw mixture and the outflow of the product streams.
The existence of backpressure is a significant limitation on the productivity of current membrane modules containing elements having long hollow fibers with small diameters. In experiments conducted by the inventor herein with fiber membrane elements made of polymethylpentene hollow fibers having an internal diameter of 14 micrometers and a wall thickness of 10 micrometers, it was found that only a small extent of the fiber membrane length, about 5 centimeters, is effective in the separation process. The efficiency of separation drops further with use of thinner capillaries or with highly permeable capillaries with asymmetric wall structures.
Therefore, current membrane modules employing elements with long hollow fibers having small diameters operate at a level of productivity too low for them to be viable commercial products for meeting current uses. Thus, a need remains for improvements in the design of fiber membrane elements which will provide modules capable of operating at acceptable levels of separation productivity and of techniques for fabricating such membrane elements on a cost-effective basis.